1. Field of the Invention
The present invention relates to thermal neutron decay time logging and, more particularly, to novel methods and apparatus for producing thermal neutron decay time measurements which more accurately represent the intrinsic values of thermal neutron decay characteristics of earth formations.
2. Description of Prior Art
In well logging applications, thermal neutron decay time logs provide measurements of the rate of absorption of thermal neutrons in the earth formations traversed by a borehole. The basic physics of the thermal neutron decay time measurement are straightforward; the thermal neutron decay time (.tau.) is measured by detecting gamma rays produced by thermal neutron capture. This quantity can be converted to the formation macroscopic capture cross section as follows: EQU .SIGMA.(capture units)=(4550/.tau.)
where .tau. is the thermal neutron decay time (microseconds) and 1 capture unit=10.sup.-3 cm.sup.-1. Since chlorine is the strongest neutron absorber of common earth elements, .tau. is determined largely by the sodium chloride present in the formation water. Hence the decay time log, although useful in open hole logging, finds its principal application in cased wells for differentiating between oil or gas-bearing formations and saltwater-bearing formations, and for monitoring changes in water saturation during the production life of a well.
Thermal neutron decay time logging is provided commercially by the Schlumberger Well Services division of the Schlumberger Technology Corporation, assignee of the present application, under the mark TDT and is a widely accepted and important logging service. In providing this service, Schlumberger employs basically two logging systems, the TDT-K system which is described in U.S. Pat. No. 3,890,501 and the TDT-M system which is described in U.S. Pat. Nos. 4,223,218 and 4,224,516. In both systems, measurements of the thermal neutron decay time are made by first repetitively irradiating the formation with pulses or bursts of high energy neutrons. After each pulse, the rate at which the thermal neutron population decreases is determined by measuring gamma rays produced from capture of thermal neutrons by elements present in the formation and the borehole environment. Two detectors are provided in the tool so that measurements can be made at near and far spacings from the neutron source.
To make quantitative oil saturation determinations using thermal neutron decay time measurements, it is important to take into account how the measured (apparent) neutron decay time of a formation is related to the true neutron decay time implied by the intrinsic capture cross section of the formation. Essentially, two factors can affect the measurement. The first is the spatial diffusion of neutrons from regions of high neutron density to regions of low neutron density, and arises from the fact that the source of neutrons is a point source. The second factor is the presence in the borehole of materials such as the logging tool itself, borehole fluid, casing and cement, all of which typically will have capture cross sections different from the formation capture cross section. Various proposals have been made over the years to correct for these effects. The aforementioned U.S. Pat. No. 3,890,501, for example, describes several alternative techniques for correcting the near spacing detector measurements of .tau. and .SIGMA. (.tau..sub.N and .SIGMA..sub.N) for diffusion effects to afford values more accurately reflecting the intrinsic .tau. and .SIGMA. (.tau..sub.INT and .SIGMA..sub.INT), including (1) the use of diffusion departure (correction) curves and (2) multiplying .SIGMA. by the .tau..sub.N /.tau..sub.F ratio or a ratio of far detector count rates. The '501 patent also discloses that the far spacing detector .tau. and .SIGMA. measurements (.tau..sub.F and .SIGMA..sub.F) are less effected by neutron diffusion than the near detector measurements .tau..sub.N and .SIGMA..sub.N and that, if desired, the .tau..sub.F measurement can be used in certain circumstances as a diffusion-corrected measurement of .tau..sub.INT. These diffusion-correction techniques have been used with success in the TDT-K system. More recently, the TDT-M system has been developed which employs sixteen detection time gates to make more effective use of the far-spacing detector in deriving .tau. and .SIGMA. measurements, as described in the aforementioned '218 and '516 patents.
Because the TDT log is such an important service, however, it is desirable to improve the service still further, and in particular more fully to correct the measured values of .tau. and .SIGMA. for environmental effects, including both diffusion effects and borehole capture cross section effects.